1. Field of the Invention
The present invention relates to a reflection type liquid crystal display device of a normally white mode, which employs a thin film transistor (referred to hereinafter as TFT) and a reflective display electrode connected to the TFT.
2. Description of the Related Art
In recent years, effort has been directed towards research and development of a reflection type liquid crystal display device with a reflective display electrode in which an image is displayed by reflecting incident light from the observer side.
A reflection type liquid crystal display device of a normally white mode using a conventional TFT is described below.
In the present application, a xe2x80x9cnormallyxe2x80x9d white mode refers to the liquid crystal orientation mode in which light is transmitted when no voltage is applied to the liquid crystal.
A reflection type liquid crystal display device is a display device in which an image is displayed by reflecting incident light from the observer side with a reflective display electrode.
FIG. 1 is a plan view showing an area around a display pixel region in a conventional reflection type liquid crystal display device. FIG. 2 is a cross-sectional view taken along line Bxe2x80x94B in FIG. 1.
As shown in FIG. 1, a gate signal line 51 which includes gate electrodes 11 in portions thereof is provided for supplying a gate signal to a gate. A drain signal line (data signal line) 52 which includes drain electrodes 16 in portions thereof is provided for supplying a drain signal to a drain. A TFT is provided near an intersection of the gate signal line and the drain signal line. In the TFT, the gate 11 is connected to the gate signal line 51, the drain 13d is connected to the drain signal line 52, and the source 13s is connected to a reflective display electrode 20.
As shown in FIGS. 1 and 2, sequentially provided on an insulator substrate 10 made of a material such as quartz glass or non-alkali glass are first gate electrodes 11 composed of a refractory metal such as Cr or Mo, a gate insulating film 12 composed of an SiN film and SiO2 film, and an active layer 13 formed with poly-silicon film in discrete island patterns.
The active layer 13 includes channels 13c provided above the first gate electrodes 11. The active layer 13 further includes a source 13s and a drain 13d formed by ion doping on the respective sides of the channels 13c. 
A stopper insulating film 14 made of SiO2 film is provided over the channels 13c to function as the mask covering the channels 13c such that ions do not enter into the channels 13c during ion doping.
Furthermore, an interlayer insulating film 15 is formed on the entire surface over the gate insulating film 12, the active layer 13, and the stopper insulating film 14, by sequential lamination of a SiO2 film, a SiN film, and a SiO2 film.
A contact hole formed in the interlayer insulating film 15 in a position corresponding to the drain 13d is filled with metal sing Al only, or by sequentially depositing Mo and Al, to thereby form a drain electrode 16.
The drain signal line 52 is disposed on the interlayer insulating film 15. Furthermore, a planarizing insulating film 19 made of a material such as an organic resin is provided on the entire surface.
As shown in FIG. 2, a contact hole is formed in the planarizing insulating film 19 in a position corresponding to the source 13s. A reflective display electrode 20 that contacts the source 13s through this contact hole is formed using a reflective and conductive material such as Al. The reflective display electrode 20 simultaneously serves as a source electrode. An alignment layer 21 for orienting the liquid crystal 36 is provided further on top.
A counter electrode substrate 30 has, on the side facing the insulator substrate 10 and the liquid crystal 36, color filters 31 for each of red (R), green (G), and blue (B), a counter electrode 32, and an alignment layer 33. Provided on the other side of the substrate 30 are a retardation film 34 and a polarizer 35. The insulator substrate 10 provided with TFTs in the above-described manner and the counter electrode substrate 30 are sealed by surrounding the substrates with a sealing adhesive. The gap created between the two substrates is then filled with liquid crystal 36 to complete the liquid crystal display device.
According to a conventional reflection type liquid crystal display device such as that described above, the reflective display electrodes 20 are arranged such that the gaps between adjacent electrodes 20 lie above the gate signal lines 51 and the drain signal lines 52, as shown in FIG. 1. In the example of FIG. 2, referring to a drain signal line 52 having drain electrodes as portions thereof, a gap between adjacent reflective display electrodes 20 is arranged above the drain signal line 52.
In this arrangement, incident light 101 from a light source (the side of an observer 100) transmits through the polarizer 35, the retardation film 34, the counter electrode substrate 30, the alignment layer 33, the liquid crystal 36, the alignment layer 21, and the planarizing insulating film 19 to reach the drain signal line 52. The incident light 101 is then reflected by the drain signal line 52 through a reverse path, namely, the path indicated by a dotted line 102, to radiate out of the polarizer 35. More specifically, incident light 101 entering through the abovementioned path is reflected by the drain signal line 52 composed of Al having a reflectance of approximately 95% or more, and reflected light 102 is therefore constantly observed by the observer 100.
In this way, a conventional reflective liquid crystal display device of a normally white mode is disadvantageous in that, even when a black image is displayed, the reflected light 102 generates display defects in the form of white lines along the drain signal lines 52, thereby decreasing contrast.
A further disadvantage is that the aperture ratio is small because the reflective display electrodes 20 are not formed in areas in which the TFTs are provided.
The present invention was created in light of the above problems. The purpose of the present invention is to provide a reflection type liquid crystal display device of a normally white mode having a high aperture ratio, in which generation of display defects due to reflection of incident light by signal lines is prevented when a black image is displayed on the device.
The reflection type liquid crystal display device according to the present invention is a reflection type liquid crystal display device of a normally white mode comprising a substrate having a gate signal line and a data signal line arranged to intersect one another, and a plurality of display pixel regions defined by the gate signal line and the data signal line. Each display pixel region includes a thin film transistor connected to the gate signal line and the data signal line, and a reflective display electrode connected to the thin film transistor. The reflective display electrode covers an area in which the thin film transistor is formed, and extends into an adjacent display pixel region located beyond the gate signal line. A gap between the reflective display electrodes in the display pixel regions located adjacent to one another on either side of the gate signal line is positioned in an offset arrangement from the position in which the gate signal line is formed.
In another aspect, the reflection type liquid crystal display device according to the present invention is a reflection type liquid crystal display device of a normally white mode comprising a substrate having a gate signal line and a data signal line arranged to intersect one another, and a plurality of display pixel regions defined by the gate signal line and the data signal line, each display pixel region including a thin film transistor connected to the gate signal line and the data signal line, and a reflective display electrode connected to the thin film transistor, wherein the reflective display electrode covers an area in which the thin film transistor is formed, and extends into an adjacent display pixel region located beyond the data signal line. A gap between the reflective display electrodes in the display pixel regions located adjacent to one another on either side of the data signal line is positioned in an offset arrangement from the position in which the data signal line is formed.
In a further aspect of the present invention, the reflective display electrode covers an area in which the thin film transistor is formed, and extends into an adjacent display pixel region located beyond the gate signal line, while a gap between the reflective display electrodes in the display pixel regions located adjacent to one another on either side of the gate signal line is positioned in an offset arrangement from the position in which the gate signal line is formed, and, at the same time, the reflective display electrode also extends into an adjacent display pixel region located beyond the data signal line. The gap between the reflective display electrodes in the display pixel regions located adjacent to one another on either side of the data signal line is positioned in an offset arrangement from the position in which the data signal line is formed.
In a still further aspect of the present invention, the thin film transistor comprises a first gate electrode constituting an integral portion of the gate signal line, a semiconductor film including a first insulating film, a channel, a source, and a drain, and a second insulating film. On the second insulating film, a second gate electrode is formed so as to cover the area over the channel.
In another aspect of the present invention, the second gate electrode is connected to the first gate electrode.
In a further aspect of the present invention, the thin film transistor comprises a plurality of first gate electrodes constituting integral portions of the gate signal line, a first insulating film, a semiconductor film extending so as to intersect said plurality of first gate electrodes and including channels formed in positions overlapping each of said plurality of first gate electrodes, a second insulating film, and a second gate electrodes formed on the second insulating film so as to cover over an area in which the channel is formed.
As described above, a gap between reflective display electrodes in adjacent display pixel regions is positioned in an offset arrangement from the position of a gate signal line and/or, data-line which is often composed of a conductive material having a high reflectance. In this way, when displaying a black image on the reflection type display device of a normally white mode, generation of display defects showing white lines along the signal lines caused by the reflection of incident light by the signal lines can be prevented. Accordingly, a reflection type display device having high contrast and a high aperture rate can be achieved.
Further, by configuring the thin film transistor with a first gate electrode and a second gate electrode sandwiching a semiconductor film having insulating films and a channel, influence of the electric field generated by reflective display electrode for the channel can be reliably prevented, accomplishing a display device having minimal fluctuation in the characteristics of the thin film transistors.
Still further, the width of the second gate electrode in the channel length direction is made narrower than the width of the first gate electrode in the same channel length direction. With to this arrangement, the effective channel length is prevented from becoming longer than the target channel length due to mask misalignment or other causes-during fabrication of the thin film transistor. In addition, generation of leak current in the semiconductor film is prevented, which may otherwise be caused when the peripheral portions of the second gate electrode overlap the channel end portions. In this way, the structure of the present invention minimizes variance of display characteristics in each display region and each display device.